This invention relates to a heat-sensitive stencil and to a method of fabricating same.
One known heat-sensitive stencil is composed of an ink-permeable thin paper serving as an ink support and a thermoplastic resin film bonded with an adhesive to the support. The stencil is heated imagewise by, for example, a thermal head to perforate the heated portions of the thermoplastic resin film, thereby obtaining a printing master for reproducing images by mimeographic printing. The conventional stencil, however, poses problems because (1) the adhesive tends to be accumulated in interstices between fibers to form xe2x80x9cfinsxe2x80x9d which prevent the thermal perforation during the master forming step and the passage of an ink during the printing step, (2) the fibers per se prevent smooth passage of an ink and (3) the paper support is relatively expensive.
To cope with the above problems, JP-A-54-33117 proposes a stencil having no paper support and composed substantially only of a thermoplastic resin film. While this stencil can completely solve the above-mentioned problems, a new serious problem arises; i.e. it is necessary to significantly increase the thickness of the stencil in order to obtain satisfactory stiffness required for transferring the stencil master during printing stage. An increase of the thickness results in the lowering of the thermal sensitivity.
JP-A 62-198459 discloses a method of fabricating a stencil wherein a multiplicity of closed patterns such as circular patterns are formed by gravure printing of a radiation-curable heat-resisting resin on a thermoplastic resin film, followed by curing. Since this method unavoidably gives patterns have a thickness of 50 xcexcm or more, the formation of perforations with a thermal head is not easy. Further, ink stains are apt to be formed on prints obtained using such a stencil master.
It is, therefore, an object of the present invention to provide a heat-sensitive stencil having satisfactory stiffness and excellent sensitivity to thermal perforation.
Another object of the present invention is to provide a heat-sensitive stencil without an adhesive and paper.
It is a further object of the present invention to provide a heat-sensitive stencil of the above-mentioned type which can give a printing master capable of producing uniform, clear printings even with a small amount of an ink.
It is yet a further object of the present invention to provide a heat-sensitive stencil of the above-mentioned type which can give a printing master capable of producing printed images free of ink blurs and stains.
It is a further object of the present invention to provide a method which can easily fabricate a heat-sensitive stencil.
In accomplishing the foregoing objects, there is provided in accordance with one aspect of the present invention a heat-sensitive stencil comprising a thermoplastic resin film, and a porous resin layer formed thereon, said stencil having an air permeability in the range of 1.0 cm3/cm2 sec to 157 cm3/cm2xc2x7sec in a portion thereof when said thermoplastic resin film of said portion is perforated to form perforations providing an open ratio SO/SP of at least 0.2, wherein SO represents a total area of said perforations and SP represents the area of said portion.
The provision of the porous layer can impart satisfactory stiffness to the stencil without adversely affecting the sensitivity to perforation thereof.
The air permeability of a printing master has been found to represent the ink permeability thereof. An air permeability of a printing master of below 1.0 cm3/cm2xc2x7sec even with an open ratio SO/SP of 1.0 means that the porous resin layer has a low porosity and that the master has poor ink permeability. On the other hand, an air permeability in excess of 157 cm3/cm2xc2x7sec means that the porous resin layer has poor ink retentivity so that stains and blurs are apt to be formed.
In another aspect, the present invention provides a method of preparing a heat-sensitive stencil, comprising the steps of:
(a) dissolving a resin in a mixed solvent including a first solvent capable of dissolving said resin, and a second solvent substantially incapable of dissolving said resin and having an evaporation rate lower than that of said first solvent, thereby to obtain a coating liquid;
(b) applying said coating liquid over a surface of a thermoplastic resin film to form a resin coating; and
(c) heating said resin coating to dryness.
The present invention also provides a method of preparing a heat-sensitive stencil, comprising the steps of:
(a) applying a solution of a resin in a first solvent over a surface of a thermoplastic resin film to form a wet resin coating over said surface;
(b) spraying vapors or fine droplets of a second solvent substantially incapable of dissolving said resin over said wet resin coating so that said second solvent is taken into said wet resin coating to cause a portion of said resin to precipitate; and
(c) heating said resin coating to dryness.